This invention relates to reducing noise caused by the rotation of an element having an outside surface in the shape of a polygon, such as an octagonal mirror.
Rotatable mirrors are commonly used in laser printers. The mirrors are a series of flat mirror surfaces combined to form a regular polygon. Each of the flat surfaces, as it rotates, directs a laser beam reflected form the surface to sweep across a photoconductor. This is widely practiced.
This invention is directed to reducing the noise caused by the air driven by the corners of the polygon. Such noise has a strong pure tone (single frequency component). A previous invention, shown in U.S. Pat. No. 5,901,000, that is assigned to the assignee of this invention, employs a circular wall surrounding the mirror except for an opening permitting the light beam to reach the mirror and be reflected by the mirror to the optical system. That invention employs a depending rib to influence the airflow pattern and thereby reduce noise.
For purposes of this invention the mirror motor system and mounting may be the same as in the foregoing patent 5,901,000. This invention, however, does not employ the rib. Also, in the foregoing patent 5,901,000 the wall was suspended from a top cover. This invention fixes the wall to the bottom support.
In normal operation a polygonal mirror is rotated rapidly around the center of the polygon at one, predetermined velocity. A circular wall surrounding the mirror is fixed to a bottom support plate for the mirror and extends upward from that bottom support plate. The wall has an opening that permits light to reach the polygonal surface of the mirror and to be reflected so as to sweep across a surface such as a photoconductor. The wall has outwardly extending flanges, which contact the support surface except over a limited region in which the support surface is populated with electronic components. More than 200 degrees of the region surrounding the wall does have contact with the support surface. Finally, a major area of a top over the walls contains a sound absorbing material.